Unexpected warning: ''.'' should not be used as a source

还没有人认领这个 Issue。

I often have a use-case when `"."` is a valid and correct source of my mapping. It is officially supported mapping source, so why the plugin reports it as "should not be used"? Is there any particular reason or maybe it is just a historical burden from the times when MapStruct actually didn't support `"."` as a source?

Example:

```java
record Failure(String code, String detail) {}

class Foo {
Failure failure;
}

class FooDto {
String failureCode;
String failureDetail;
}

public abstract class FooMapper {

@Mapping(target = "failure", source = ".")
public abstract Foo fromDto(FooDto fooDto);

protected Failure mapFailure(FooDto fooDto) {
return new Failure(fooDto.getFailureCode(), fooDto.getFailureDetail());
}

}
```

Of course, in this particular case we can just use:

```java
@Mapping(target = "failure.code", source = "failureCode")
@Mapping(target = "failure.detail", source = "failureDetail")
```

But there are more complex cases when the target object should be created manually, by a custom code, and the only possible source is the whole source object.

首先使用 FooMapper 示例重现该警告，并检查 MapStruct IntelliJ plugin 如何处理 source 为 "." 的 @Mapping 值。将该行为与 MapStruct 文档中对这种 source 形式的支持进行比较；当有效映射不再被错误地警告，或已明确确定警告的原因时，即可完成。

由索引模型根据 Issue 内容生成。